The image forming apparatus is provided on an installing surface for the apparatus, such as a floor, at, e.g., four places by a member such as casters. However, in the case where flatness of the installing surface is not good, even when installing portions are thus provided at four places, the image forming apparatus is stable in a plane formed by three points. In this case and when the rigidity of the structure of the image forming apparatus, constituted by a frame or the like, is insufficient, a portion of the structure which is not supported goes down, so that there is a possibility that flexure and twisting are generated in the structure. Further, even in a constitution in which the image forming apparatus is installed on the installing surface at three places, the load acting on respective points contacting the installing surface varies depending on the flatness of the installing surface, so that there is the possibility that flexure and twisting are generated in the structure. When the flexure and the twisting thus generate in the structure, accuracy of respective portions for carrying out image formation lowers, so that there is a possibility that the lowering in accuracy has an influence on an image quality.
Therefore, a constitution in which rigidity of a bottom of a structure in which an installing portion for installing the apparatus on an installing surface is formed has been conventionally proposed. For example, one in which the bottom is not constituted by a single metal plate but is constituted in a box type by combining two or more metal plates has been proposed (Japanese Laid-Open Patent Application (JP-A 2002-217560). Further, also a constitution in which the bottom is formed by connecting a base reinforcing member, prepared by connecting intermediary members in a diagonal line shape, with a base body (JP-A 2005-221905).